Injury of most raw fruits and vegetables by methods of post-harvest minimal processing, such as peeling, cutting, slicing, crushing, etc., results in wound response by the injured tissue, including enzymatic browning. Enzymatic browning results from the polyphenol oxidase-catalyzed oxidation of phenolic compounds to ortho-quinones, which sequentially polymerize to form dark-colored pigments. Cellular disruption caused by minimal processing allows the interaction of polyphenol oxidase and phenolic compounds, which are compartmentalized in normal, intact cells.
Heretofore, most commercial efforts to control enzymatic browning in raw, minimally processed fruits and vegetables have involved the use of sulfite treatments. However, sulfites have been implicated as the cause of allergic reactions in certain sensitive individuals, resulting in anaphylactic shock and death in rare cases. (Taylor, S. L., N. A. Higley, and R. K. Bush, 1986, Sulfites in foods: uses, analytical methods, residues, fate, exposure assessment, metabolism, toxicity, and hypersensitivity. Adv. Food Res. 30: 1-76.)
Virtually all sulfite alternatives for the prevention of enzymatic browning in raw, peeled potatoes that have been patented or reported in the literature involve the use of acidic solutions with a pH lower than 3. Such treatments generally involve the use of organic acids such as citric and/or malic acid in combination with ascorbic or erythorbic acid. The pH of these dips is much lower than the optimum pH for polyphenol oxidase activity. Adjuncts such as thiol compounds (e.g. L-cysteine) may be added to acidic solutions to improve their efficacy against browning.
Unfortunately, most of these treatments do not provide adequate shelf life, and low-pH solutions tend to induce "case hardening," which is a surface firming observable in the cooked product, resulting in what is often described as "raw-like" lumps when the potatoes are mashed. The case-hardened layer often separates from the rest of the potato tissue during cooking.
Sapers and Miller found that pre-treatment of peeled potatoes in heated organic acid solutions greatly improved the efficacy of conventional, organic acid-based browning inhibitor dips. (Sapers, G. M., and R. L. Miller, 1995, Heated ascorbic/citric acid solution as browning inhibitor for pre-peeled potatoes. J. Food Sci. 60 (4): 762-766 & 776.) However, this treatment induced even more textural deficiencies in the final, cooked product than did the acidic browning inhibitor dips, alone.
It is thought that acidic dips induce case hardening by mobilizing and redistributing divalent cations from the potato cell interiors. (Sapers, G. M., P. H. Cooke, R. L. Miller, A. E. Heidel, and S. T. Martin, Structural changes related to textural deficiencies of pre-peeled potatoes. Presented at the 1996 IFT Annual Meeting, New Orleans, La., June 26.) The cations cross-link partially de-methylated pectin molecules in the cell walls and middle lamella, which renders the tissue more resistant to thermal degradation. Heating could increase this effect by activation of endogenous pectin methylesterase, resulting in further de-esterification of the pectin and the creation of more binding sites for the calcium or magnesium. (Bartolome, L. G., and J. E. Hoff, 1972, Firming of potatoes: biochemical effects of pre-heating.) Heating may also increase hardening by promoting greater diffusion of the acids into the potato tissue.
There is currently a need for a method of treatment for fresh, peeled potatoes that provides adequate protection from enzymatic browning without the use of sulfites, and which limits textural deficiencies in the product following cooking.